Structural changes in the membrane of cells infected respectively with scrapie, visna and herpes viruses have been identified using freeze-fracture, surface replication and immunolabeling techniques. In the scrapie brain, abnormal features were observed in the neural and astrocytic membranes of clinically affected animals. In visna infected cells, the virus maturation was characterized by the insertion and growth of surface units probably representing viral antigens, by the reoganization of the host membrane proteins under these units and by the attachment of a coiled capsid to the modified membrane. In herpes infection, unusual groups of tightly packed particles were identified in the cleaved membrane at the sites where viral antigen is probably $ incorporated.